Biological indicators of relative sea level change during the last 2 ka on the islands of Vis, Ravnik and Biševo in Croatia

Abstract

On the particularly exposed sites on rocky coasts of islands Vis, Ravnik and Bisevo in Croatia, biogenic littoral rims built by the coralline rhodophyte Lithophyllum lichenoides can be found. The presence of thick and well-developed Lithophyllum rims, considered to be precise (±10 cm) sea-level indicator, points directly to the rising sea-level environment. Up to now five locations have been studied: 3 on the island of Vis, 1 on Ravnik and 1 on the island of Bisevo. Biogenic rims were mapped, measured and sampled for 14C dating. Three of the researched algal structures have two dominant levels: the upper one, 6 m long and 40 to 150 cm wide, and the lower one with a maximal extension of 40 cm. The surface of the lower level is 60 cm below the recent one. The thickness of each level at its most protruding part is around 15 cm. Today, on the relatively horizontal surface of the upper level algal rims the process of biodestruction prevails over bioconstruction. Living talli of Lithophyllum lichenoides were found only in small patches at the highest position on the horizontal surface of the rim and along its upper limit. Such a situation could clearly be connected to the recent acceleration of the sea-level rise as mentioned by Laborel et al., 1994. The prograding morphology of the algal rims points to the slowing of the sea-level rise starting at around −1 m below the MSL, when the algal rims began to develop. Based on the archaeological markers from 2300 ± 100 yr BP till around 1900 ± 100 yr BP the rate of relative sea level change was around 0.89 mm/yr (Faivre et al., in press) and then, according to the 14C dated algal samples it slowed down to 0.45 mm/yr from around 1450 yr BP till today. The obtained results have been compared not only with archaeological markers, e.g. submerged port remains of the ancient Issa harbour on the island of Vis, but also with geomorphological sea-level markers, as well as with other available data along the Croatian coast.